Switching

Switching - Topics rconne cting LAN se e gm nts I nte r)...

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Csci 232 Computer Networks Switching and Forwarding 1 Topics Interconnecting LAN segments HUB (Physical Layer) Bridge (Link layer) Layer 2 Switch (multi-port bridge, link layer) Interconnecting networks Layer 3 Switch (network layer) Router (network layer) ATM Networks
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Csci 232 Computer Networks Switching and Forwarding 2 Interconnecting LAN Segments (Repeating) Hubs (layer 1 devices) Bridges (layer 2 devices) Basic Functions Self learning and bridge forwarding table Forwarding/filtering algorithm Bridge looping problem and spanning tree algorithm Ethernet Switches Remark: switches are essentially multi-port bridges. What we say about bridges also holds for switches! Readings Section 3.2
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Csci 232 Computer Networks Switching and Forwarding 3 Interconnecting with Hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain if a node in CS and a node EE transmit at same time: collision Encoding is different: Manchester vs. 4B/5B Recreates each bit, boosts its energy strength, and transmits the bit to all other interfaces
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Csci 232 Computer Networks Switching and Forwarding 4 Bridges Link layer device stores and forwards Ethernet frames examines frame header and selectively forwards frame based on MAC destination address -- filtering when frame is to be forwarded on a LAN segment, uses CSMA/CD to access the LAN segment transparent hosts are unaware of the presence of bridges plug-and-play, self-learning bridges do not need to be configured
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Csci 232 Computer Networks Switching and Forwarding 5 Bridges: Traffic Isolation Bridge installation breaks LAN into LAN segments Bridges filter packets: same-LAN-segment frames not usually forwarded onto other LAN segments segments become separate collision domains bridge collision domain collision domain = hub = host LAN (IP network) LAN segment LAN segment
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Csci 232 Computer Networks Switching and Forwarding 6 Forwarding How to determine to which LAN segment to forward frame?
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Csci 232 Computer Networks Switching and Forwarding 7 Self Learning A bridge has a bridge (forwarding) table Entry in bridge forwarding table: <Node LAN Address, Bridge Interface, Time Stamp> stale entries in table dropped (TTL can be 60 min) Bridges learn which hosts can be reached through which interfaces when frame received, bridge “learns” location of sender: incoming LAN segment records sender/location pair in bridge forwarding table
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Csci 232 Computer Networks Switching and Forwarding 8 Filtering/Forwarding When bridge receives a frame: index bridge table using dest MAC address if entry found for destination then{ if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived
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Switching - Topics rconne cting LAN se e gm nts I nte r)...

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